The invention relates to a resonant microwave sensor for determining properties of a material, that is to be examined, by means of the high-frequency measurement of a reflection factor, having
a microwave supply conductor for feeding the high-frequency signal, and
a sensor waveguide that is coupled to the microwave supply conductor.
The measurement of material properties with the aid of microwave sensors by evaluating resonant frequencies and the quality of a resonance curve that is picked up by applying a swept-frequency high-frequency signal to the microwave sensor is sufficiently well known. In this case, the signal is coupled into the microwave sensor with a variable frequency, and the resonant frequency and, if appropriate, the quality, are determined.
Thus, German Utility Model 297 16 639 U1 describes a microwave stray-field sensor for measuring moisture and/or density, in the case of which a moist dielectric material is inserted into the resonator, and the density and humidity of the material are determined by shifting the resonant frequency. In order to avoid scattering losses that falsify the measurement result, it is proposed that the wavelength at the site of the generation of the resonance signal be substantially lower than in the free space of the resonator.
The resonator is constructed as a wire loop that is surrounded by a thin dielectric. In another embodiment, the resonator is formed from a circular dielectric ceramic body, the microwaves being coupled into the resonator via coaxial lines and capacitively active coupling pins.
These embodiments are likewise disclosed in EP 0 908 718 A1.
U.S. Pat. No. 3,946,308 discloses a microwave sensor for measuring moisture that has a dielectric resonator with a metallic conductor, a solid dielectric material, and an inlet antenna and an outlet antenna.
Furthermore, DE-A 24 54 788 A1 discloses a method and a device for determining the humidity of a gaseous medium. The basic method for measuring the humidity of a medium with the aid of the circuit quality of a resonance curve is to be gathered from this printed publication. With this method, a microwave oscillation of variable frequency, that is to say a swept-frequency signal, is coupled into the resonator and coupled out again separately therefrom. The amplitudes of the coupled-out oscillations are measured and recorded as a function of the frequency at constant amplitudes of the coupled-in oscillations.
In addition to these methods, in which the measurement information is obtained from the reflection at the material to be examined, methods are known in which the variation in the electromagnetic waves is evaluated during transmission through a material.
The problem of the known resonant microwave sensors that are based on the reflection method consists in scattering losses, broadbandedness and size. Moreover, the known microwave sensors cannot be integrated in optimum fashion in structures for in-situ measurement of characteristic material properties.
The object of the invention was therefore to create an improved generic resonant microwave sensor.
The object is achieved by means of a helical conductor that is arranged inside the sensor waveguide.
The use of the helical conductor as resonant metallic helix permits the construction of a compact sensor waveguide as resonator. Moreover, the resonant frequency of the microwave sensor can be set with the aid of the helical conductor.
The sensor waveguide as resonator is preferably a cylindrical tube, the microwave supply conductor being arranged, for example, as a coaxial hollow conductor at a first end face of the sensor waveguide. The second end face of the sensor waveguide is open, and so the material to be examined can penetrate into the resonator. There is a resultant variation in the dielectric properties of the filling in the sensor waveguide, and in the characteristic quantities of the resonator, that is to say the resonant frequency and quality are detuned. The properties of the material to be examined such as, for example, the relative humidity, can then be determined in a known way with the aid of these characteristic quantities.
The resonant frequency and quality of the sensor waveguide are calculated from the reflection factor R.
The helical conductor preferably extends in the longitudinal direction of the sensor waveguide.
The helical conductor is preferably held by a support and centered in the sensor waveguide.
For coupling purposes, an intermediate layer is advantageously provided between the first end face of the sensor waveguide and the helical conductor. The thickness of the intermediate layer determines the coupling factor. The quality of the sensor can be set with the aid of the intermediate layer. In order to enhance the measurement dynamics, the thickness of the intermediate layer should be selected so as to achieve the greatest possible coupling.
The resonant frequency of the microwave sensor is largely determined by the dimension of the helical conductor.
It is advantageous, furthermore, when an additional coupling layer is provided between the helical conductor and the material to be examined or a sensitive layer in the region of the second end face of the sensor waveguide. The thickness of the coupling layer determines the coupling of the helical conductor to the material to be examined, and thus the measurement sensitivity.
In the method for measuring properties of a material with the aid of a resonant microwave sensor described above, the frequency of the swept-frequency signal for exciting the microwave sensor should be lower than the cutoff frequency of the microwave sensor.
The length of the sensor waveguide should in this case be large enough to avoid emission to the outside and thus scattering losses.
The support and the intermediate layer are preferably designed in one piece.
It is particularly advantageous when the intermediate layer and/or the support consist of Teflon.